Number of Rhizobia and delayed inoculation influence nodulation of clovers

The relationship between numbers of rhizobia and nodulation response of legumes is of considerable practical importance. Experiments were done under controlled conditions to determine the influence of numbers of Rhizobium leguminosarum biovar. trifolii on nodulation of arrowleaf clover (Trifolium vesiculosum Savi.) and crimson clover (T. incarnatum L.). Numbers of rhizobia in excess of 1000 per seed did not substantially increase earliness of nodulation or total number of nodules formed on the taproot. Nodules, however, were formed nearer the top of the taproot as numbers of rhizobia increased to 100,000 per seed. Delayed inoculation experiments indicated that nodulation sites for these clovers only remained susceptible to infection for less than 1 day. Delaying inoculation for 4 days resulted in only a 1 to 2 day delay in nodulation for arrowleaf and crimson clovers respectively and no delay for subterranean clover (T. subterraneum L.). Apparently, larger seedlings nodulated faster.